Federal prosecutors say more arrests could come soon in their investigation of a large-scale college admissions bribery and cheating ring, with a number of defendants already charged in a sealed complaint.

But an unspecified number of other defendants already face charges that federal prosecutors say will be unveiled "in the near future." Prosecutors typically ask that charges against suspects who haven't been arrested remain under seal.

U.S. Attorney Andrew Lelling has signaled from the beginning that their investigation is ongoing.

William "Rick" Singer leaves federal court in Boston after being charged with racketeering conspiracy, money laundering conspiracy, conspiracy to defraud the United States, and obstruction of justice on March 12.(Photo: Scott Eisen, Getty Images)

Affidavits filed in the cases suggest that the scope of college cheating went far beyond the 33 parents already charged.

► Singer said he's used some version of the scam to get almost 800 students into prestigious colleges and universities. "So I did 761 what I would call, 'side doors,' he told one parent last year.

► Prosecutors say Singer earned more than $25 million from the scheme. But the amounts accounted for in court filings total less than $6 million of that income.

► The earliest criminal activity alleged in court filings dates back to 2008, but most of the cases charged are from admissions in just the last two years.

It's likely that not everyone who participated in the scheme will wind up facing charges. The statute of limitations for most fraud and bribery offenses is five years. Criminal tax fraud charges can go back six years, based on when the return is filed, but there's no limit on civil penalties.

And the evidence is strongest in cases from last year because the FBI had more than three months of court-authorized wiretaps in which neither Singer nor the parents knew they were being recorded.

After that warrant expired last September, the FBI had Singer call parents back – one after the other – to get them to incriminate themselves on tape. But that gambit seemed to run its course last month when one parent who used Singer three years ago seemed to catch on to Singer's cooperation and denied everything.

Federal prosecutors in Boston, now obliged to turn over evidence to defendants, have requested gag orders forbidding them from talking about what's in those documents. That's partly because so much of the evidence relies on sensitive financial records, grades and test scores – even psychological evaluations of some of the students.

But also, prosecutors said, "the discovery materials further include information concerning uncharged co-conspirators and targets of the investigation who have not yet been publicly charged."

Even more specifically, they've asked for an extension until at least May 31 to provide the court with an inventory of all wiretaps used in the case because the investigation is continuing.

"The Government further expects, in the near future, to charge and arrest (redacted) through a criminal complaint that will be unsealed upon the arrest of the charged defendants," Assistant U.S. Attorney Eric Rosen said in a court filing.

There could be several reasons those charges are still sealed, said Mark MacDougall, a former federal prosecutor who's now a white-collar defense attorney at Akin Gump.

"It’s possible people have been charged and it’s under seal because they’re cooperating in an unrelated case. But that gets complicated," he said. "I think what’s more likely is you have someone who's overseas."

MacDougall said he's skeptical that there will be another wholesale round of dozens of indictments.

For one, prosecutors likely picked the strongest cases. With Singer facing the most prison time of anybody, a jury is unlikely to take his word on other cases without independent evidence like audio recordings.

So if there are parents who worked with Singer and are worried they could still be charged, MacDougall had this advice: "Be cool," he said. "The last thing I would be doing is calling the prosecutors in Boston."

Felicity Huffman, left, and Lori Loughlin outside of federal court in Boston on April 3, where they faced charges in a nationwide college admissions bribery scandal.(Photo: AP)

Dozens of calls

The Varsity Blues cases are unusual because the FBI caught ringleader Singer first, and used him to incriminate the parents – like getting the drug dealer to flip on his customers instead of the other way around.

That's how prosecutors gathered evidence against actresses Lori Loughlin and Felicity Huffman. While they obtained some emails, most of the evidence consists of phone calls with Singer last fall. In those calls, recorded by the FBI, Singer had them concoct a cover story in case they were questioned about the payments by the IRS – which prosecutors submitted as evidence of guilt.

Singer made dozens of those calls. But with the investigation dragging on and college acceptance letters coming every day, prosecutors may have decided to wrap up the case because they were getting diminishing returns.

The week before the FBI unsealed the indictments, one parent seemed to get wise.

Singer had been systematically calling parents while the FBI listened in, hoping to get them to implicate themselves. But the first Sunday in March, Singer called Stephen Semprevivo, a 53-year-old Los Angeles sales executive, under the pretext that he was being audited by the IRS. Semprevivo wouldn't bite.

"You know, all I know is that we – you know, we used you for the charity stuff and we used you for the counseling, and your dealings are your dealings," he told Singer.

"No I get that," Singer said. "And I understand that, but at the same time we were all a part of ..."

"No, I don’t agree with that at all," Semprevivo said. "You know, you did what you did, (Rick), and that was your stuff. Okay?"

Two days later, Singer signed a plea agreement, effectively wrapping up his cooperation. One week after that, Semprevivo was indicted. He has since agreed to plead guilty to mail fraud and serve 18 months in prison.

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Amid the major college admissions bribery scandal that's spelled legal trouble for actresses Lori Loughlin and Felicity Huffman, a lot of people in Hollywood have weighed in with their own thoughts. Scroll through to see what Ben Stiller, Lena Dunham, the cast of "Full House" and more stars had to say about the controversy. Frazer Harrison, Getty Images

Felicity Huffman pleaded guilty and issued an apology in an April statement to USA TODAY: "I am in full acceptance of my guilt, and with deep regret and shame over what I have done, I accept full responsibility for my actions and will accept the consequences that stem from those actions. I am ashamed of the pain I have caused my daughter, my family, my friends, my colleagues and the educational community. I want to apologize to them and, especially, I want to apologize to the students who work hard every day to get into college, and to their parents who make tremendous sacrifices to support their children and do so honestly." KATHERINE TAYLOR, EPA-EFE

Kim Kardasian West said in an April 20 interview with Van Jones that she wants to keep her kids "as grounded as possible," and shared her dismay toward the idea of buying their way into anything. "If they couldn’t get into a school, I would never want to use privilege to try to force them into a situation that they wouldn’t thrive in any way ... that is not appropriate," she said. Evan Agostini, Invision

Candace Cameron Bure spoke out in support of Loughlin, her "Full House" co-star, while accepting a Kids' Choice Award for the spinoff "Fuller House." "A loving family sticks together no matter what," she said onstage. "They stick together through the hard times, they support each other, they encourage one another, they pray for each other and they stand by their side, no matter how tough it gets." CHRIS DELMAS, AFP/Getty Images

"Full House" patriarch Bob Saget broke his silence to TMZ with a diplomatic answer. "I just love who I love. Candace said it really good at the Kids’ Choice Awards. You love who you love," he said, adding, "Life’s complicated. I don’t like talking about it." Kevork Djansezian, Getty Images

Fellow "Full House" actor John Stamos, who was with Saget when TMZ questioned him, said he didn't want to comment. "I will at some point, I'm just not ready yet," he said. Frederick M. Brown/Getty Images

Ben Stiller joked during an appearance on "The Ellen DeGeneres Show" that he had taken some extraordinary measures to help daughter Ella on her college search: "It's great, she's going to Yale on a full football scholarship. And she's going to major in Photoshopping." He later added that he wants "to do anything I can to support her. You know, legally." Jamie McCarthy, Getty Images

In a since-deleted Instagram post, producer and rapper Dr. Dre had a cheeky caption to celebrate his daughter's college acceptance in late March: "My daughter got accepted into USC all on her own. No jail time!!!" Kevin Winter, Getty Images

After the controversy was dubbed "Operation Varsity Blues," James Van Der Beek chimed in with a reference to the 1999 movie he starred in.
"If only there was a succinct turn of phrase these kids could have used to inform their parents they were not desirous of their life path..." Van Der Beek joked the day the story broke, referencing the "Varsity Blues" scene in which his character told his father he didn't want to play football in college.
"Playing football at West Canaan may have been the opportunity of your lifetime, but I don't want your life," his character says. Dan MacMedan/USA TODAY NETWORK

The official account for "Full Frontal With Samantha Bee" had a sarcastic take: "Breaking news: Entrance into college unjust, tilted in favor of white kids with rich parents." Evan Agostini/Invision/AP

"The irony will be that these parents spent all this money to hustle into top universities and are now in the middle of this (expletive) show just to find out in a few years that their kids only have dreams of being an influencer," Olivia Munn tweeted. Charley Gallay, Getty Images for Starz

"All the people involved in this college scam should have gathered their money and started a small elite college where Lori Loughlin teaches a class on smiling," Lena Dunham tweeted. "My parents didn't care enough about college to scam, but they'd definitely buy me a boyfriend who is willing to sit with me at the ER." Jordan Strauss, Invision/AP